Crankcase Conundrum

Both cases are from 750 models, the case(s) on the left (208115) are from a '72 Combat, the case(s0 on the right (220455) are from a '73 with a standard tune. Top set, note that the left (Combat) has the timing chamber drain back hole plugged and that a new hole drilled near the bottom of the oil pump mount. On the right the drain back hole (high) is functional, which would make it seem like an oil level that is (unnecessarily??) high.

Bottom set, note the holes drilled in the oil gallery (left picture), you can also see the timing chamber drain hole more clearly. The case on the right side of the lower picture set has none of these features.

It seems to me that the Combat cases (left side, top and bottom) have some good fixes, the question is: Should I modify the case whose picture is on the right in a similar fashion? The bigger question is which set is correct?

I would drill the new drainback hole in the later model case and use it.
A screen for the sump pump is a good feature. jim

comnoz said:

I would drill the new drainback hole in the later model case and use it.
A screen for the sump pump is a good feature. jim

Click to expand...

What screen ??
They are both combat breather cases.
Do you offer a retrofit sump screen conversion for combat cases?

dynodave said:

comnoz said:

I would drill the new drainback hole in the later model case and use it.
A screen for the sump pump is a good feature. jim

Click to expand...

Do you offer a retrofit sump screen conversion for combat cases?

Click to expand...

Now that would be something to think about. There are a lot of Combats out there and some mods that are good and bad at the same time. Fixing the forward hole on the timing case but creating other problems when hogging out the drive side case without a filtering screen for big bits floating in the crank.
Cheers,
Thomas

dynodave said:

comnoz said:

I would drill the new drainback hole in the later model case and use it.
A screen for the sump pump is a good feature. jim

Click to expand...

What screen ??
They are both combat breather cases.
Do you offer a retrofit sump screen conversion for combat cases?

Click to expand...

No, I hadn't expanded the picture so I could see the RH cases and just figured it was 73 cases with a sump plug.

So no big difference other than the RH case has the better type of cam bushing in it.

RoadScholar said:

Both cases are from 750 models, the case(s) on the left (208115) are from a '72 Combat, the case(s0 on the right (220455) are from a '73 with a standard tune. Top set, note that the left (Combat) has the timing chamber drain back hole plugged and that a new hole drilled near the bottom of the oil pump mount. On the right the drain back hole (high) is functional, which would make it seem like an oil level that is (unnecessarily??) high.

Bottom set, note the holes drilled in the oil gallery (left picture), you can also see the timing chamber drain hole more clearly. The case on the right side of the lower picture set has none of these features.

It seems to me that the Combat cases (left side, top and bottom) have some good fixes, the question is: Should I modify the case whose picture is on the right in a similar fashion? The bigger question is which set is correct?

Click to expand...

Are these 7 addition drillings into the gallery ( bottom left picture) necessary & recommended

Any thoughts please??

The seven holes in the left side case are ample but large enough to allow the oil pump to digest fatal bits of metal.

The photos below were originally posted by Windy in this thread: https://www.accessnorton.com/NortonCommando/combat-fixes.10709/page-3



I have found comment by Dyno Dave that he uses even smaller holes but I failed to find photos.

I would be curious if anyone has ever added secondary magnetic drain plug to one of these cases to move the magnet closer to the oil pickup? Is there enough meat and space to squeeze one in? Wouldn't be as good as a screen but maybe better than a magnet sitting forward.

Doing it this way since around 2004 or so..
keep the small holes small and low in the trough...
Plus a little horizontal half groove in both cases for a central hole at the very bottom
All the small holes add up to a larger size than the original (plugged with epoxy)
Just remember...you will never suck out the oil any LOWER than the HIGHEST hole.

Thanks for posting that Dave.

I have to admit that I have never completely understood why the forward hole needs to be plugged. Would it not make sense to open it back up with a tiny hole such as the ones being drilled toward the rear?

rvich said:

Thanks for posting that Dave.

I have to admit that I have never completely understood why the forward hole needs to be plugged. Would it not make sense to open it back up with a tiny hole such as the ones being drilled toward the rear?

Click to expand...

Just remember...you will never suck out the oil any LOWER than the HIGHEST hole.
The rear of the crank case will perpetually stay full of oil

dynodave said:

Just remember...you will never suck out the oil any LOWER than the HIGHEST hole.
The rear of the crank case will perpetually stay full of oil

Click to expand...

That assumes you are drawing 100% of the scavenge pump's capacity thru a single hole...

I understand the reasoning..... I don't intend to be racing & I doubt using high speed or high engine revs will happen either, so what are the risks of retaining front scavenge port....
And what is really needed is some sort of filter screen instalation over the port (s)

If you watch the oil return to your tank and start increasing RPMs, you will see that a stock Combat case will continue to return oil until the revs are near 5k. Presumably this wet sumping situation starts to present itself at a lower RPM to a lesser degree. You would have to keep the RPMs in this range long enough to starve the feed side of the oil pump for this to become an issue. There isn't a lot of reason to have a Combat cam unless you use that range, so this combination is problematic. For those of us who just visit that area while shifting it isn't a huge problem. The scavenge side of the pump catches up to a great enough degree when RPMs drop to keep oil flowing to the tank. It has been demonstrated that a well placed reed valve can help return this oil to the tank where it can feed the oil pump. The reed valve alone might be the best thing a Combat owner could do for a stock case. As for the size of the pick up holes, if any bits of your internals break off, you don't want them going to the pump and destroying it. Some of the cam bushings were known for this and an updated bushing helps. But the smaller holes offer a screen of sorts to protect the pump. Ideally, you could remove a screen for inspection.

I assume that if it were as simple as machining a threaded hole and screwing in a screened pickup that somebody would have done it by now.

Now for the disclaimer, I do not know how long you can hold a stock Combat motor at that RPM level before you starve it for oil. But if you typically ride at RPM levels below 4k and don't have pieces break off inside your motor, you can ride around on a stock Combat without issue.

This topic does not address main bearing selection which of course was another problem with the original motors.

I suspect that if you use the breather mount on the back of the case to mount a reed valve that the pickup location becomes a moot point, but I have never seen the experts support this idea.

All of Dave's holes are drilled on the same horizontal line, so, if air is pulled, there's no oil in the sump. Further, as he said, their combined area exceeds that of the original single hole.

rvich said:

Now for the disclaimer, I do not know how long you can hold a stock Combat motor at that RPM level before you starve it for oil. But if you typically ride at RPM levels below 4k and don't have pieces break off inside your motor, you can ride around on a stock Combat without issue.

Click to expand...

Fred is a big proponent of the Combat case mods after his racing experience. On the street you would never hold those rpms too long in most cases. An occasional blast to the higher rpms, sure, but sustained? Probably not.

here is what I did to mine. On the other side the little stainless extension sticks down into the case where I carved it out.

swooshdave said:

Fred is a big proponent of the Combat case mods after his racing experience. On the street you would never hold those rpms too long in most cases. An occasional blast to the higher rpms, sure, but sustained? Probably not.

Click to expand...

My Combat cases are unmodified. The day of tear down is coming. I am undecided as to what I will be doing once inside, but I do think that a reed valve is more important than the pickup. However, the idea of straining out any stray bits before they get to the pump can't be completely ignored. But yes, I agree that for most of us, we aren't going to wet sump the motor from prolonged high RPMs. Although with the 19-tooth sprocket at 70 MPH highway speeds you might be able to do it.

(The fact that I have an unmodified Combat is one of the reasons that I like to keep track of what mods are being done. I am in fact undecided on the fate of the head and cam. But that is not the topic of this thread!)

It all a bit like the food in our holiday hotel.....decisions decisions.
Of all the possible mods available the one that alludes us it the one of a strainer for the scavenge inlet
IMHO for what i will be using her for the most important. And after stripping to find a trashed oil pump by a suspected piece of follower screw lock wire.... a fix to this matter is needed.

I have a potential scheme in my head but will not work on it until it comes to the top of the project queue. Plan is to machine an aluminium block with an added sump filter screen (from GY6) and access plug (M30 1.5 pitch thread), mill flat the surface of the 72 driveside crankcase and also a dome shaped hole to take the crown of a dome shaped filter screen, then after bolting on the new sump plate mill a hole from of the side filter cavity angled to bisect the feedhole to the scavenge side of the pump. Downside is the driveside case is machined, upside is a filter screen that is cleanable on regular basis.